1. Field of the Invention
The present invention relates generally to the control of fiber optic gyroscopes, and more particularly to a method and apparatus to suppress color noise in fiber optic gyroscopes.
2. Background of the Invention
Inertial rotation sensors, for determining orientation and/or rate-of-turn with respect to an inertial frame of reference, are important elements of attitude and heading reference systems used by navigable vehicles such as aircraft. For a long period of time, such orientation and rate-of-turn determinations have typically been made using spinning mass gyroscopes. Progress in the field has resulted in many refinements and the development of various types of gyroscopes suited to specific applications. In recent years, fiber optical gyroscopes have emerged as a significant improvement over the typical spinning mass gyroscopes.
A fiber optic gyroscope is typically constructed using a loop of fiber optic material that guides counter-propagating light waves that are traveling within the fiber optic loop. After traversing the loop, the counter-propagating waves are combined so that they constructively or destructively interfere to form an optical output signal. The intensity of the optical output signal varies as a function of the interference, which is dependent upon the relative phase of the counter-propagating waves. From this information, determinations regarding the orientation and/or rate-of-turn with respect to an inertial frame of reference can be derived.
A closed loop rotation sensor feeds a signal indicative of the Sagnac phase shift to an apparatus for adjusting the phase of the counter-propagating waves to nullify the rotation-induced phase difference between them. The amount that the waves must be adjusted in either phase to nullify the Sagnac phase shift indicates the rotation rate of the sensing loop.
In order to be suitable for inertial navigation applications, a rotation sensor must have a relatively wide dynamic range. The typical rotation sensor is capable of detecting rotation rates as low as 0.001 degrees per hour and as high as 1,000 degrees per second. The dynamic range, ratio of the upper and the finest resolution, measured by a typical rotation sensor, is approximately nine orders of magnitude or 109.
Closed loop fiber optic rotation sensors are attractive due to the increase in the scale factor stability and linearity. Additionally, closed loop operation is feasible due to the availability of high-speed components such as integrated optics phase modulators. Such phase modulators are effective for providing the desired amount of phase modulation for measuring rotation rates in the required dynamic range. However, certain voltage-dependent errors in the feedback signal, phase servo, or electrical cross-coupling, can all cause the servo loop to become less stable at certain rotation rates.
In particular, the system becomes less stable at or near a zero input rate, where the fiber optic rotation sensor output is non-linear with the input rate. Typically, the loop closure electronics feedback circuit will settle at a point where the feedback-dependent voltage error cancels the rate induced Sagnac phase shift and the sensor output signal will be zero for a finite input rate. This range of rates where the fiber optic gyroscope output rate is zero for finite rate range is known as the “dead band,” “dead zone,” and “region of instability.” Other rates at where possible output errors may occur depend upon the modulation/demodulation techniques used in processing the output of the fiber optic rotation sensor.
In addition to the “dead band” performance issue, the present inventors have discovered that the feedback signal sent to the phase modulator to null the rate-induced Sagnac phase shift has a repeated pattern corresponding to a fixed input rate. This repeated pattern is synchronous to the reset frequency of the feedback signal to maintain itself within the maximum range of the driving circuit. Due to the nonlinearity of the driving circuit and the imperfection of IOC nonlinear and time-dependent capacitive and resistive characteristics, the feedback signal renders a rate output with the frequency content directly related to the reset frequency. This rate dependent sinusoids (RDS, but also referred to herein as “color noise” or “fly-back noise”) becomes a performance limiting factor if its amplitude is larger than the angle random noise or the bias instability.
In view of the foregoing, it should be appreciated that it would desirable to provide a fiber optic gyroscope that is less susceptible to performance limitations associated with the color noise phenomenon. It should also be appreciated that it would be desirable to provide a method and apparatus for improving the performance stability of inertial guidance systems that incorporate fiber optic gyroscopes.